Hydraulic drives are employed in many working vehicles, such as agricultural vehicles and harvesting machines. Such a hydraulic drive comprises a pump driven by an internal combustion engine, which pump is hydraulically connected (by a hydraulic fluid) to a hydraulic motor which drives one or more wheels. In many vehicles, wheels of the front and rear axles are driven by respective hydraulic motors, with one or more such motors serving the wheels of a given axle. Often with such motors, the displacement volume of the hydraulic motor is variable, as is the displacement volume of the associated pump.
Such a vehicle is disclosed in EP 1,223,069 A. An internal combustion engine drives the adjustable hydraulic pump, which pump is hydraulically connected (by a hydraulic fluid) to the adjustable hydraulic motors, each of which motors serves to drive one axle. The rotational speed of each motor and the pressure at the inlet and outlet of the motor are measured. An electronic control unit is connected to a control lever, and controls the swash plates of the hydraulic motors and hydraulic pump. According to this reference, the traverse angles of the hydraulic pump and the hydraulic motors are adjusted in accordance with the speed setting of the control lever. If the rotational speeds of the axles mutually differ, which is suggestive of slipping, the hydraulic motor associated with the slipping is adjusted. The other hydraulic motor(s) and the hydraulic pump are adjusted in order to maintain a constant speed of travel. The manner of control of the hydraulic pump is not further described.
The underlying problem of the present invention is deemed to be to devise a drive system for a working vehicle, which system provides improved control of the hydraulic pump and hydraulic motors. In addition it is sought to provide means of improved braking.